1. Field of the Invention
The present invention relates to a method of producing straight-chain acrylonitrile dimers. More particularly, the present invention relates to a method of producing straight-chain acrylonitrile dimers including 1,4-dicyanobutene, 1,4-dicyanobutadiene and adiponitrile in the presence of a ruthenium catalyst at a high selectivity.
The above-mentioned straight-chain acrylonitrile dimers are useful as an intermediate for producing hexamethylene-diamine (which is an important material for producing nylon 66), rust-inhibiting agents and vulcanization promoters for rubber materials.
2. Description of Related Art
A method of producing 1,4-dicyanobutene compounds and adiponitrile by using a catalyst comprising a ruthenium compound is well-known from A. Misono et al., Bull. Chem. Soc. Jpn., 41 (1968), 396-401. In this method, acrylonitrile is dimerized in the presence of a ruthenium catalyst in an atmosphere containing hydrogen gas, Misono et al., disclose that if this method is carried out without using the hydrogen gas atmosphere, no dimerization of acrylonitrile occurs, and the dimerization of acrylonitrile can proceed only within the hydrogen-containing gas atmosphere. This method is, however, disadvantageous in that acrylonitrile is brought into contact with the ruthenium catalyst in the presence of hydrogen, wherein dimerization of acrylonitrile proceeds and simultaneously hydrogenation of acrylonitrile, which is not desired, occurs as a side reaction and a large amount of propionitrile is produced as a by-product.
JP-B-44-24,585, JP-B-45-4,048 and JP-B-54-12,450 disclose methods of producing straight-chain acrylonitrile dimers with an enhanced selectivity in the presence of hydrogen. In these methods, the selectivity of the straight-chain acrylonitrile dimers is 55 to 67%. Nevertheless, since the dimerization of acrylonitrile in these methods is carried out in the presence of hydrogen, it is unavoidable that propionitrile be produced as a by-product at a high selectivity of 33 to 45%.
JP-A-51-146,420 discloses a method of dimerizing acrylonitrile at a reaction temperature of 300.degree. C. to 600.degree. C. in the presence of a ruthenium catalyst in the absence of hydrogen. Nevertheless, this method is disadvantageous in that to restrict the by-production of propionitrile and to enhance the selectivity of acrylonitrile to the target straight-chain dimers thereof it is necessary to restrict the conversion of acrylonitrile to the dimers to a level of several percent. When the conversion is enhanced, the by-production of the undesired propionitrile is increased and the selectivity of acrylonitrile to the target dimers is reduced. Further, in this case, the resultant dinitrile product comprises a mixture of straight-chain dimers and branched-chain dimers. Accordingly, the method of the Japanese publication is unsatisfactory.
As mentioned above, the conventional methods for dimerizing acrylonitrile are disadvantageous in that a by-product, for example, propionitrile, which is difficult to return to acrylonitrile, is produced in a large amount, and thus the selectivity of acrylonitrile to the target dimers is reduced, or in that the resultant dimerization product consists of a mixture of straight-chain dimers and branched-chain dimers and thus the selectivity to the target straight-chain dimers becomes low, or in that the use of hydrogen causes an undesirable high risk of explosion when the reaction system is mixed with air or oxygen, and thus specific means for ensuring complete safety for the reaction apparatus and procedures is necessary, and thus the reaction apparatus and procedures become complicated and costly.